Among the litany of violations at U.S. nuclear power plants are missing or mishandled nuclear material, inadequate emergency plans, faulty backup power generators, corroded cooling pipes and even marijuana use inside a nuclear plant, according to an ABC News review of four years of Nuclear Regulatory Commission safety records.

And perhaps most troubling of all, critics say, the commission has failed to correct the violations in a timely fashion.

"The Nuclear Regulatory Commission has very good safety regulations but they have very bad enforcement of those regulations," said David Lochbaum, a nuclear scientist with the nonprofit Union of Concerned Scientists.

There are 104 U.S. nuclear power plants, producing 20 percent of the country's electricity at world-class safety levels, according to the Nuclear Energy Institute.

And the Nuclear Regulatory Commission has reported no abnormal occurrences to Congress in the past five years, the institute said in a statement.

Nevertheless, Lochbaum and the Union of Concerned Scientists found 14 "near misses" at nuclear plants in 2010. And there were 56 serious violations at nuclear power plants from 2007 to 2011, according the ABC News review of NRC records.

At the Dresden Nuclear Power Plant in Illinois, for instance, which is located within 50 miles of the 7 million people who live in and around Chicago, nuclear material went missing in 2007. The Nuclear Regulatory Commission fined the operator -- Exelon Corp. -- after discovering the facility had failed to "keep complete records showing the inventory [and] disposal of all special nuclear material in its possession."

As a result, two fuel pellets and equipment with nuclear material could not be accounted for.

Exelon did not contest the violation and paid the fine, a company spokesman said. "We took the learnings from that violation with respect to ways we can improve our spent-fuel practices," Marshall Murphy said.

Two years later, federal regulators cited Dresden for allowing unlicensed operators to work with radioactive control rods. The workers allowed three control rods to be moved out of the core. When alarms went off, workers initially ignored them.

Murphy said the company concurred with the NRC's determination. "
We have also taken a number of steps to ensure a similar event would not occur at any of our sites and shared the lessons from that with the industry," he said.

"In both violations, neither employees or the public were ever jeopardized, but we take them seriously, we always look to learn from them, and we do that going forward.

Still, Lochbaum of the Union of Concerned Scientists said, "This event is disturbing. In August 1997, the NRC issued information ? about a reactivity mismanagement problem at Exelon's Zion nuclear plant," which was retired the following year.

"It was an epoch event in the industry in that other plants owners noted it and took steps to address [the issue]. Yet, a decade later, Exelon's Dresden plant experiences an eerily similar repetition of the control-room operator problems."

Nuclear Regulatory Commission Safety Records

The lost material was almost certainly shipped to a licensed, low-level waste disposal site, Lochbaum said.

At the Indian Point nuclear plant just outside New York City, the NRC found that an earthquake safety device has been leaking for 18 years.

In the event of an earthquake, Lochbaum said, the faulty safety device would not help prevent water from leaking out of the reactor. A lack of water to cool the fuel rods has been the most critical problem at the Fukushima plant in Japan after the recent earthquake and tsunami.

"The NRC has known it's been leaking since 1993," Lochbaum said, "but they've done nothing to fix it."

While declining to address specific violations, Roger Hanna, a spokesman for the NRC, said "we do require plant to comply, and we do follow up for corrections" when violations are discovered.

But NRC records examined by ABC News show that such incidents are not uncommon:

In June 2009, at the Southern Nuclear Operating Co. Inc. in Birmingham, Ala., the emergency diesel generator -- which would be used in the event of a disaster -- was deemed inoperable, after years of neglect.

"Cracks in the glands of the emergency diesel generator couplings had been observed since 1988, but the licensee did not recognize the cracking was an indication of coupling deterioration," according to the NRC report.

On April 19, 2010, the NRC cited the Tennessee Valley Authority Browns Ferry nuclear plant near Decatur for failing to provide "fire protection features capable of limiting fire damage."

The NRC fire protection regulations in effect today were developed as a direct result of the Browns Ferry fire on March 22, 1975.

In June 2010, Duke Energy, operators of the William McGuire nuclear plant in Mecklenburg County, N.C., was cited by the NRC after a contract employee was caught using marijuana inside the protected area.

NRC safety records show that inadequate emergency planning was a recurring problem in the industry from 2007 to 2011. Violations included unapproved emergency plans and plan changes, inadequate fire planning and precautions, falsified "fire watch" certification sheets," inadequate flooding precautions, an insufficient tone alert radio system to notify the populace in a potential emergency and faulty assessment of containment barrier thresholds.

Corroded water pipes and cooling problems were also recurring issues.

The National Energy Institute respnse

In a statement, they said the NRC reports to Congress in the last five years show no abnormal occurrences at U.S. nuclear plants. In fact,

they continued, America's reactors produce 20 percent of all electricity at world class safety levels.